Diamond Annual Review 2019/20

56 57 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 Imaging andMicroscopy Group Beamline I13 (Coherence Branchline I13-1) Understanding the environmental implications of the Fukushima nuclear accident Related publications: Martin P. G., Louvel M., Cipiccia S., Jones C. P., Batey D. J., HallamK. R., Yang I. A. X., SatouY., Rau C., Mosselmans J. F.W., Richards D. A. & Scott T. B. Provenance of uraniumparticulate contained within Fukushima Daiichi Nuclear Power Plant Unit 1 ejecta material. Nat. Commun. 10 , 2801 (2019). DOI: 10.1038/s41467-019-10937-z Martin P. G., Jones C. P., Cipiccia S., Batey D. J., HallamK. R., SatouY., Griffiths I., Rau C., Richards D. A., Sueki K., Ishii T. & Scott T. B. Compositional and structural analysis of Fukushima-derived particulates using high-resolution x-ray imaging and synchrotron characterisation techniques. Sci Rep. 10 , 1636 (2020). DOI: 10.1038/s41598-020-58545-y Publication keywords: Fukushima; Uranium; Nuclear; Contamination; Radiation; SIMS; Synchrotron; Tomography T he Fukushima Daiichi Nuclear Power Plant accident released a large number of radioactive particulates into the environment. To understand the environmental and health hazards thismaterial poses, scientists fromthe University of Bristol studied the physical form and chemical composition of small (sub-mm) released particles. The plant operators also needed to knowwhat exactly happened in the reactors, and how the physical properties of thematerial would affect removal and reprocessing/waste storage operations. Diamond Light Source's X-ray Imaging and Coherence Beamline (I13-1) was the ideal platform through which to perform high-resolution imagingtoexaminethefineinternalstructureoftheparticulatesandtoperformelementalmappingtodeterminethematerial’scomposition. The results show that the particulates have a glassy composition that is likely to be stable in the environment. Uranium in the samples demonstrates that fuel was released from the reactor core. The researchers were able to establish a chronology of the release events, providing information on the likely state of the ‘fuel debris’ – facts critical for retrieval and reactor decommissioning. Experiments at Diamond have analysed two of the three particulate types released from Fukushima, and an upcoming investigation on the Microfocus Beamline (I18) will examine the third. Such is the impact of this work that this Bristol and Diamond team have been asked to analysematerial released obtained from the Chernobyl exclusion zone – the first work to do so in nearly 20 years. The accident at Japan’s Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011 is one of only two International Nuclear Event Scale (INES) Level 7 incidents – the most severe, to have ever occurred (the only other being Chernobyl). Initiated by the Magnitude 9.0 Great Tohoku earthquake off the countries eastern coast, the ensuing 15 m high tsunami subsequently crippled the plants routine and emergency systems that provided core cooling to each of the sites four operational reactors. Despite the efforts of the plants operators, the Tokyo Electric Power Company (TEPCO), to bring the reactors under control through emergency seawater injection, the decay heat in each of the reactors progressively rose over the following week to dangerous levels. Eventually, the heat and gaseous build-up in each of the reactors and reactor buildings became too great, with a series of explosions (Unit 1 and Unit 3) and discrete releases (Unit 2) occurring up to 10 days after the initial event. Although approximately 80% of the 520 PBq of radioactivity 1 (approximately 10% of Chernobyl) released from Fukushima was dispersed out into the Pacific Ocean 2 , a considerable inventory of radioactive material was deposited over the Japanese land area. This contamination existed as a number of isotopically distinct yet spatially mixed plumes, composed primarily of the volatile fission product caesium (Cs). While the vast majority of the work to-date has focused on quantifying the distribution and environmental implications of this easy to detect ionic (gaseous) radiocaesium release, work at the University of Bristol and Diamond has sought to study the particulate input into environment and the elements (e.g. actinides) more commonly overlooked as they are much harder to detect. Such knowledge will allow for a greater understanding of the potential health implications associatedwith other material released into the environment in addition to crucial insight into the release dynamics and the current state of material in the reactor – essential for the soon to commence fuel debris retrieval operations. X-ray tomography (XRT) combined with elemental distribution information derived from X-ray fluorescence (XRF) is capable of detailing both the internal structure of the sub-mm particulate collected from the land surrounding the plant. The bright, strongly coherent and highly focused beam of X-rays produced by the synchrotron enables rapid data collection from short exposure times, with the unique optical configuration on I13-1 allowing for image resolutions not attainable on other beamlines. Following their collection and isolation from bulk sediments by collaborators at the Japan Atomic Energy Agency (JAEA) 3 , the radioactive particulate was encapsulated within a special X-ray transparent kapton tape. This would prevent the sample from being dislodged during the analysis and being lost on the beamline. The suite of sub-mm particulate studied on I13-1 was sourced exclusively from reactor Unit 1 that sustained a large reactor building hydrogen explosion but dispersed radioactive material closer to the plant boundary than the smaller particles from Unit 2. For each particle, following the careful alignment of the beamline optics, an X-ray tomography (absorption contrast) scan was undertaken by taking an X-ray ‘photo’ as the sample was slowly rotated within the beam – producing an image like a hospital X-ray. By then using tomographic reconstruction algorithms, it was possible to produce an absorption 3D representation of each particle, detailing its internal form (Fig 1). The highly coherent X-rays produced on I13-1 also allowed for an enhanced examination of the samples fine structure using X-ray ptychography, which showed the highly porous and fibrous nature of the particulate samples. Following the physical structural analysis of each sample with XRT, an analysis of the composition was performed using XRF. Each particle was rastered through the beam path with an elemental spectrum obtained for each pixel‘step’. Reconstructions of the species distribution, paired with the XRT derived structure, were then performed using software developed in-house at Diamond 4 . Using the combined XRT, ptychography and XRF analysis of the samples, it was possible to understand how the material from reactor Unit 1 at the FDNPP was formed and the conditions in the reactor at the time of its release. Most importantly, this work has shown for the first time that uranium derived from the reactor Unit 1 core was released during the accident –with core integrity and containment becoming sufficiently compromised during the accident. In fact, the presence of both cement and stainless-steel within some of the particles highlights the explosive nature of the release event (Fig. 2). This knowledge of the likely state of the current conditions in the reactor is important as the Japanese plan to shortly commence operations to remove the damaged core debris material in Units 1, 2 and 3 at the FDNPP. The close collaboration between the University of Bristol, JAEA and Diamond hasprovidedcrucial informationtoguideJapaneseauthorities intheremediation of the contaminated environment and in on-site decommissioning activities.This forensic analysis has opened the door to future in-depth high resolution studies on radioactive particulate material derived from not just the FDNPP accident - from the mm to the nm scale. References: 1. Steinhauser G. et al. Comparison of the Chernobyl and Fukushima nuclear accidents: a review of the environmental impacts. Sci Total Environ. 470-471 , 800-817 (2014). DOI: 10.1016/j.scitotenv.2013.10.029 2. Stohl A. et al. Xenon-133 and caesium-137 releases into the atmosphere from the Fukushima Dai-ichi nuclear power plant: determination of the source term, atmospheric dispersion, and deposition. Atmos. Chem. Phys . 12 , 2313-2343 (2012). DOI: 10.5194/acp-12-2313-2012 3. SatouY. et al. First successful isolation of radioactive particles from soil near the Fukushima Daiichi Nuclear Power Plant. Anthropocene 14 , 71-76 (2016). DOI: 10.1016/j.ancene.2016.05.001 4. Cipiccia S. et al. An Iterative Self-Absorption Correction Algorithm for 3D Ptycho-Fluorescence Imaging. Microsc. Microanal. 24 (S2), 94-95 (2018). DOI: 10.1017/S1431927618012862 Funding acknowledgement: Daiwa Anglo-Japanese Foundation (Grant Reference: 11424), Great Britain Sasakawa Foundation (Grant Reference: 5223), EPSRC (Reference: EP/ S020659/1), EPSRC (Reference: EP/K040340/1) Corresponding author: Dr Peter Martin, University of Bristol, peter.martin@bristol.ac.uk Figure 1: Tomographic reconstruction of one of the particulate samples analysed. A high-density iron-based fragment is observed embedded into the surface of the material. The particle is approximately 500 µm in length. Figure 2: Tomographic slice through the particle overlain with compositional data derived from XRF analysis. Orange = stainless-steel, Green = cement. Scale bars = 100 μm. White boxes = locations where voids connect, Yellow box and * = location at which two bubbles have likely fused. Figure 3: Tomographic sections through an FNDPP derived particle, with uranium inclusions identified using XRF overlain.